The invention relates to a process for preparing a fluorine-doped tin oxide in the form of a fine powder. A frequent technical requirement is to provide electrically insulating materials with an anti-static or electrically conductive coating in order to prevent electrostatic charges from accumulating on the material or in order to make the material suitable for a specific use. The coating must usually satisfy the requirement of having sufficiently high electrical conductivity under extreme conditions, particularly under very dry conditions. An additional requirement is frequently that the coating should be transparent. This latter requirement applies, for example, to conductive coatings of exposure windows or exposure cylinders of copying machines or original scanners, to transparent film materials and to electrically conductive colored toner powders which have been given the required conductivity by the application of an electrically conductive coating to the surface of the toner powder particles.
Transparent electrically conductive or anti-static coatings can be formed by applying a metal, metal oxide or metal salt, e.g., gold, chromium, nickel, palladium, indium oxide, stannic oxide or cupro-iodide, in the form of a thin film to the insulating surface, e.g. by sputtering or vapor-coating in a vacuum. These methods, however, are expensive and in many cases do not give the required good coating adhesion. Moreover, these coating methods are not always applicable, for example, they are not usable for giving fine toner powder particles an electrically conductive layer.
U.S. Pat. No. 4,431,764 describes compositions for forming transparent electrically conductive coatings comprising a film-forming binder and finely divided tin oxide having a particle size less than 0.4 micrometer, the tin oxide being doped with 0.1 to 20% by weight of antimony, in the form of Sb.sub.2 O.sub.3 or Sb.sub.2 O.sub.5.
The tin oxide described in that patent can also be used to form conductive layers without the aid of a film-forming binder, by softening the (thermoplastic) surface of the area to be coated, softening being carried out either by heat or by means of a solvent or solvent vapor, and by covering the softened and thus tacky surface with the tin oxide. For example, electrically conductive toner powder can be made by softening the toner powder particles in a hot gas stream and bringing the fine tin oxide powder into the gas stream, or by intensively mixing a mixture of toner powder and fine tin oxide in a ball mill for some time so that the toner powder particles soften as a result of the frictional heat produced and the tin oxide particles are deposited on the softened surface.
Disadvantages of antimony-doped tin oxide in accordance with U.S. Pat. No. 4,431,764 are that its electrical resistance is still relatively high, and that the dry powder itself is not loose but has fairly poor flow properties, so that it is difficult to divide it finely. Conductive toner powders made with this tin oxide contain a fairly considerable amount of loose tin oxide or tin oxide insufficiently bonded to the toner powder, so that considerable soiling of the apparatus in which the toner powder is used occurs.